This invention relates generally to the sensing of fluid vapors. More particularly, this invention relates to the sensing of vapors from alkyl esters of phosphoric acid which are present in hydraulic fluids.
Alkyl esters of phosphoric acid, and particularly, tributylphosphate, are common components of hydraulic fluids especially of the type used in aircrafts. As the vapors of such components may be toxic, there is a need for a simple and reliable sensor to measure the presence of such vapors in the ambient air, particularly at stratetic locations within an aircraft so that any leakage of the vapor into ambient air can be detected and appropriate measures be taken before any adverse effect upon personnel within the aircraft.
The sensor should be capable of measuring a concentration of such vapors in the air as low as 400 parts per billion by volume. In addition, the sensor should be capable of forming very rapid analysis (less than 3 minutes) and require little routine service and maintenance over a long period of time.
The sensor also must be selective to the particular alkyl ester of phosphoric acid present in the hydraulic fluid. Accordingly, the sensing process must not be interfered with by any other trace chemical impurities that may be present in the ambient air, particularly that of the aircraft. Several of the potential chemicals which could present interference with the sensor include air (N.sub.2 and O.sub.2), water, sulfur dioxide, ozone, nitromethane, nitromethane, hydrogen chloride, and carbon dioxide.
The monitoring of such vapors, especially tributylphosphate, presents many obstacles. Sampling of tributylphosphate vapor in the air under normal conditions is virtually impossible since it absorbs on the surface of the sampling system and requires considerable time to completely desorb. Moreover, tributylphosphate has a very high boiling point (269.degree. C.), at which temperature it decomposes.